An ultrasonic imaging apparatus for transmitting/receiving pulse ultrasonic waves to/from a living body to capture an image of the inside of the living body is widely used for medical diagnosis.
In the fields of X rays and MRI among image diagnosis modalities, a contrast medium has been being used for capturing an image of a blood vessel and the like. The purpose is to obtain a contrast-enhanced image of a structure and a distribution of a blood vessel system by injecting a contrast medium into blood and to carry out a diagnosis of a disease reflected in the blood vessel system such as a malignant tumor or an infarct with high accuracy.
In contrast, a contrast medium has not been widely used for ultrasonic diagnosis. In recent years, contrast media are being widely used because of appearance of a contrast medium in the form of a formulation obtained by stabilizing microbubbles in the size of the order of microns. The principle uses the phenomenon such that microbubbles each having the diameter of about 1 micron vibrate with a large amplitude in resonance with ultrasonic waves of a few MHz used for ultrasonic diagnosis, as a result, ultrasonic waves in the frequency range are scattered well, and contrast creating performance increases.
The microbubble ultrasonic contrast medium is characterized by its nonlinearity for the following reason. Microbubbles have a nature that increase in the volume when negative pressure is applied is much larger than decrease in the volume when positive pressure having the same amplitude is applied. Consequently, echo signals scattered by the microbubbles include many second harmonic components having a frequency twice as high as that of a transmission signal. V. L. Newhouse et al. have reported a method of obtaining a blood flow Doppler signal emphasized on a soft tissue on the basis of the second harmonic components for the first time in 1992 (refer to, for example, Document 1: 1992 IEEE Ultrasonics Symposium Proceedings, pp. 1175-1177).
P. N. Burns et al. have proposed a pulse inversion of performing transmitting/receiving operations twice by using a transmission acoustic pressure pulse waveform obtained by inverting the polarity and adding obtained two echo signals (refer to, for example, Document 2: U.S. Pat. No. 6,095,980). By the addition, a fundamental wave component of an echo signal from a soft tissue whose movement can be ignored is cancelled out since a signal whose phase is turned by 180° is added. However, since a signal whose phase is turned by 360° is added, a second harmonic wave component grows double by the addition. Although the necessary number of transmission times doubles, in theory, the fundamental wave component can be eliminated from the soft tissue without a band pass filter, so that a second harmonic wave echo signal having excellent distance resolution can be obtained. With respect to a scatterer whose change occurring during the twice of transmitting/receiving operations cannot be ignored like a microbubble contrast medium in the blood flow, a fundamental wave echo signal from the scatterer is not cancelled out completely. However, it rather matches the object of today of obtaining an echo image in which a contrast medium is emphasized on a soft tissue.
P. J. Phillips has proposed a method of performing transmitting/receiving operations three times while inverting the polarity of a transmission acoustic pressure pulse waveform and, simultaneously, varying the amplitude (refer to, for example, Document 3: 2001 IEEE Ultrasonics Symposium Proceedings, pp. 1739-1745). In the method, transmitting/receiving operation is performed three times while modulating the transmission amplitude like 0.5, −1, and 0.5, and echo signals obtained are added. Specifically, the same pulse waveform is used for the transmission of the first and third times, and a pulse waveform obtained by inverting the phase of the pulse waveform for the first and third times and doubling the amplitude is used for the transmission of the second time. In a manner similar to the normal pulse inversion, echo signal components from a linear scatterer whose change is slow cancel each other out, and even-number harmonic components in an echo signal, which are generated by a nonlinear scatterer and nonlinear propagation are emphasized. In addition, the method is characterized that all of components including fundamental waves out of echo signal components generated by the nonlinear scatterer and nonlinear propagation are extracted by amplification modulation. Accordingly, higher sensitivity to an echo signal by the nonlinear scatterer and nonlinear propagation than that of the normal pulse inversion is expected. The higher sensitivity is realized by using the fact that dependency of nonlinear scattering using microbubbles on an incident acoustic pressure amplitude is much larger than dependency of nonlinear propagation. By the higher sensitivity, an effect of nonlinear scattering can be detected also at a transmission acoustic pressure at which an effect of the nonlinear propagation is relatively small. Thus, as compared with normal pulse inversion, a clear distinction of a contrast medium from a soft tissue can be made more easily.